RFC 8441 support: WebSockets over h2, h2c

[riptl]

Feature Request: Introduce support for WebSockets over HTTP/2 (h2 and h2c) for the server and client.

The use-case is mainly reducing the TCP connection count by leveraging HTTP/2 multiplexing. For example, a WebSocket reverse proxy could multiplex many incoming WebSockets to a single outgoing HTTP/2 connection.

Specification: RFC 8441: Bootstrapping WebSockets with HTTP/2

For the WebSocket server this requires support in ngx.req.connect.

For the WebSocket client, it's much more complicated. I think without core modifications, the only way to multiplex is by having an NGINX light thread that runs a pure-Lua HTTP/2 implementation over cosockets, and then doing cross-coroutine IPC with this thread to create WebSockets and send/receive messages. lua-resty-http2 looks like a great candidate.